This invention relates generally to integrated circuit (IC) packages, and specifically to leadframe packages and corresponding methods for their assembly.
The forming of IC packages usually require molding equipment that encapsulates an IC die in a mold compound (for example, epoxy resin) to protect it from environmental factors such as dust, heat, moisture, static electricity, and mechanical shocks. The leadframe is the foundation of the molded IC package. The two most common leadframe metals employed by the industry are nickel-iron alloy and copper alloy.
A leadframe package may use a wire bond package substrate for mounting an IC die. In a wire bond package substrate, wire bond connections provide the electrical paths for power and signal distribution from the package substrate to an IC die. Lead fingers connect the IC package to other IC packages or the printed circuit board. A wire bond package substrate typically comes with a die-attach pad (DAP) for mounting the die.
Typically, a mold set used in a molding equipment to assemble IC packages has two halves—a top half and a bottom half. These opposing halves open to receive the package substrate, and close during the molding cycle. Opposing halves that have been mated together during the molding cycle form mold cavities. Gates are small openings into these cavities through which the mold compound is injected.
Unpredictable variations in the molding process, composition of the mold compound, and the molding equipment itself sometimes cause the mold compound to flow at different rates into the top and bottom cavities of a mold set. The uneven flow of mold compound often cause the DAP to tilt or shift from its original position, resulting in an exposed DAP, exposed wires, stressed wires, package breaks, cracks, or device failures. The appearance of exposed DAP or wires on the surface of the package is also aesthetically undesirable. Functional failures or cosmetic defects tend to become more significant during the formation of slim packages such as Quad Flat Pack (QFP), Thin QFP (TQFP), and Low-profile QFP (LQFP) packages or during the formation of packages with stacked or multiple dice. In slim and stacked-die packages, the cavities for the flow of mold compound are smaller; thus, these packages are more susceptible to failures or defects caused by imbalances in the mold flow.
One type of leadframe package is shown in FIG. 1, which shows a cross-sectional view of a typical leadframe package 100. A leadframe package substrate 102 includes a DAP 104 and corresponding lead fingers 106 disposed about DAP 104. During assembly of leadframe package 100, die 108 is mounted onto DAP 104 with an adhesive (e.g., die attach epoxy). Die 108 is then electrically connected to lead fingers 106 using wire bond connections 112. Generally, DAP 104 is sized to support die 108. In one embodiment, DAP 104 is sized to include a surface area larger than that of die 108. As such, an exposed portion of DAP 104 is available around the attached die 108. Molding cap 110, which is made out of molding compound, is formed over die 108, DAP 104, wire bond connections 112, and at least a portion of lead fingers 106. The other portion of lead fingers 106 is left extending outside of molding cap 110 to establish communication with other components, for example, a printed circuit board, when the IC package is mounted onto the printed circuit board.
The injection of mold compound into an IC package during the molding process can lead to several undesirable effects in conventional IC packages. For example, an imbalanced mold flow can cause one side of the DAP to tilt downwards and the other side of the DAP to tilt upwards. The tilting of the DAP increases the height of one side of the DAP relative to the floor of the mold cavity, resulting in disproportionate vertical heights for the tilted sides of the DAP relative to the floor. An imbalanced mold flow may occur, for example, when the mold compound flows faster into one side of the mold cavity compared to the other side.
During the molding process, the flow of mold compound into the mold cavity depends on several process parameters such as the preheat time of the molding equipment, mold temperature, mold transfer pressure, and the speed of mold flow. Even a slight variation in the process parameters may cause an imbalanced mold flow, which in turn may cause the DAP to tilt or move. As such, to prevent the DAP from tilting or moving, the window for variation in the process parameters must be kept small and meticulously adhered to.
FIG. 2 shows leadframe package 200 where DAP 204 is tilted to one side of the leadframe package. As shown in FIG. 2, the tilting of DAP 204 causes die 208 that is mounted onto DAP 204 to tilt as well. Furthermore, wire bond connections 212 are stressed and might protrude out of molding cap 210 or break apart if the degree of tilting increases. Stressed wire bond connections 212 may result in unreliable or loss of connectivity between the die and the lead fingers.
FIG. 3 shows leadframe package 300 where DAP 304 is exposed through floor 314 of the mold cavity and wire bond connection 312 is exposed through molding cap 310 of the mold cavity. As shown in FIG. 3, the tilting of DAP 304 causes wire bond connection 312 to extend upwards and protrude out of the upper surface or ceiling of the molding cap 310. The tilting also causes one side of DAP 304 to extend downwards and protrude out of the lower surface or floor 314 of the mold cavity. When viewed as an assembled product, part of wire bond connection 312 will be visible at the top surface of the leadframe package, while part of DAP 304 will be visible on the bottom surface of the package. A package such as that shown in FIG. 3 will not only be rejected due to cosmetic flaws, but may also cause reliability and quality issues in the functionality of the package.
The tilting or shifting of DAP in a package substrate causes physical defects in IC packages, leading to the scrapping of the affected packages and subsequently an increase in the costs of manufacturing. Some tilting may cause physical defects that are visible to the naked eye, while some tilting may cause physical defects that are hard to detect visually or through electrical tests. Nonetheless, the performance of the IC package can be substantially affected or altered by the physical defects caused by a tilting DAP even though they are not visually detectable.
The use of multiple or stacked dice increases the occurrence of DAP tilting. For example, as more than one die is stacked above the DAP, the space that is available for the mold compound to flow during the molding process decreases. With the decrease in space, the speed at which the mold compound flows also decreases, thereby increasing the risk of imbalanced mold flow. Thus, the probability of the DAP tilting or shifting from its original position also increases. Thinner IC packages are also susceptible to DAP tilting because the reduced vertical height also reduces the speed of mold compound flow. Therefore, the severity or frequency of DAP tilting increases in stacked-die and thin IC packages. Stacked-die and thin IC package components are typically more expensive than standard components, making the cost of physical defects in these packages even more prohibitive than in standard packages.